(a) Field of the Invention
The present invention relates generally to a liquid crystal display and more particularly to a liquid crystal display operating in vertical alignment mode.
(b) Description of the Related Art
Liquid crystal displays are one of the most widely used types of flat panel display apparatuses today. A liquid crystal display typically includes two panels having electric field generating electrodes such as pixel electrodes and a common electrode, and a liquid crystal layer interposed between the two panels. The liquid crystal display displays an image by applying a voltage to the electric field generating electrodes to generate an electric field in the liquid crystal layer. The electric field determines the alignment of liquid crystal molecules in the liquid crystal layer and controls the polarization of incident light.
Among the different types of liquid crystal displays, a liquid crystal display with a vertical alignment mode has attracted much attention recently for its high contrast ratio and wide reference viewing angle. In the vertical alignment mode, liquid crystal molecules are arranged such that major axes of the liquid crystal molecules are perpendicular to the upper and lower panels when no electric field is generated. As used herein, the “reference viewing angle” denotes a viewing angle corresponding to a contrast ratio of 1:10 or a limited angle for brightness inversion between grayscales.
To widen the reference viewing angle in the liquid crystal display with a vertical alignment mode, cutouts may be formed in the electric field-generating electrodes. Also, protrusions may be formed on the electric field-generating electrodes to widen the reference viewing angle. Since the cutouts and the protrusions can be used to control the tilt directions of liquid crystal molecules, the liquid crystal molecules can be tilted in the desired directions by using the cutouts and the protrusions. This way, the wide viewing angle can be secured.
Although the liquid crystal display with a vertical alignment mode provides a wide viewing angle, there is a problem in that its side visibility is inferior in comparison to its visibility from the front. For example, in a pattern of a vertically-aligned type of liquid crystal display provided with cutouts, images in the side portions of the liquid crystal display become brighter. In more severe cases, the brightness difference between high grayscales disappears, causing a distortion of the image.
Various techniques have been proposed to solve the problems, including a technique for providing different transmittances by dividing one pixel into two sub-pixels, coupling the two sub-pixels in a capacitive manner, and providing different voltages to the two sub-pixels by directly applying a voltage to the one sub-pixel and reducing the voltage in the other sub-pixel due to capacitive coupling.
However, the above techniques are less effective in practice than in theory because the transmittances of the two sub-pixels cannot be accurately adjusted. In particular, since the transmittances of different colors of light are different from each other, different voltage combinations must be provided for different colors. However, providing of different voltage combinations to different-colored pixels cannot be implemented. An additional problem with these techniques is that, since conductive members for the capacitive coupling must be added, the aperture ratio deteriorates. Also, the transmittance decreases due to a voltage drop caused by the capacitive coupling.
A liquid crystal display that can be operated in the vertical alignment mode with a wide reference viewing angle without the above disadvantages is desired.